The need for low emissions fuels has created an increased demand for light olefins for use in alkylation, oligomerization, MTBE and ETBE synthesis processes. In addition, a low cost supply of light olefins, particularly propylene, continues to be in demand to serve as feedstock for polyolefin, particularly polypropylene production.
Fixed bed processes for light paraffin dehydrogenation have recently attracted renewed interest for increasing olefin production. However, these types of processes typically require relatively large capital investments as well as high operating costs. It is therefore advantageous to increase olefin yield using processes, which require relatively small capital investment. It is particularly advantageous to increase olefin yield in catalytic cracking processes.
U.S. Pat. No. 4,830,728 to Herbst et al. discloses a FCC unit that is operated to maximize olefin production. The FCC unit has two separate risers into which a different feed stream is introduced. The operation of the risers is designed so that a suitable catalyst will act to convert a heavy gas oil in one riser and another suitable catalyst will act to crack a lighter naphtha feed in the other riser. Conditions within the heavy gas oil riser can be modified to maximize either gasoline or olefin production. The primary means of maximizing production of the desired product is by using a catalyst that favors the production of the desired product slate.
U.S. Pat. No. 5,389,232 to Adewuyi et al. describes a FCC process in which the catalyst contains up to 90 wt. % conventional large pore cracking catalyst and an additive containing more than 3.0 wt. % ZSM-5 (a medium pore catalyst) on a pure crystal basis on an amorphous support. The patent indicates that although ZSM-5 increases C3 and C4 olefins, high temperatures degrade the effectiveness of the ZSM-5. Therefore, a temperature of 950° F. to 1100° F. (510° C. to 593° C.) in the base of the riser is quenched with light cycle oil downstream of the base to lower the temperature in the riser 10° F.-100° F. (5.6° C.-55.6° C.). The ZSM-5 and the quench increase the production of C3/C4 light olefins but there is no appreciable ethylene product.
European Patent Specifications 490,435-B and 372,632-B and European Patent Application 385,538-A describe processes for converting hydrocarbonaceous feedstocks to olefins and gasoline using fixed or moving beds. The catalysts included ZSM-5 in a matrix, which included a large proportion of alumina.
U.S. Pat. No. 5,069,776 teaches a process for the conversion of a hydrocarbonaceous feedstock by contacting the feedstock with a moving bed of a zeolite catalyst comprising a zeolite with a medium pore diameter of 0.3 to 0.7 nm, at a temperature above about 500° C. and at a residence time less than about 10 seconds. Olefins are produced with relatively little saturated gaseous hydrocarbons being formed. Also, U.S. Pat. No. 3,928,172 to Mobil teaches a process for converting hydrocarbonaceous feedstocks wherein olefins are produced by reacting said feedstock in the presence of a ZSM-5 catalyst.
A problem inherent in producing olefin products using FCC units is that the process depends on a specific catalyst balance to maximize production of light olefins while also achieving high conversion of the 650° F.+ feed components to fuel products. In addition, even if a specific catalyst balance can be maintained to maximize overall olefin production relative to fuels, olefin selectivity is generally low due to undesirable side reactions, such as extensive cracking, isomerization, aromatization and hydrogen transfer reactions. Light saturated gases produced from undesirable side reactions result in increased costs to recover the desirable light olefins. Therefore, it is desirable to maximize olefin production in a process that allows a high degree of control over the selectivity of C3 and C4 olefins while producing minimal by-products.